The bulk of the cortical neurons of the arm motor system are located in the middle third of the postcentral gyrus (Figure 202.1). The rest of the cortical neurons of the arm motor system are scattered in the frontal and parietal areas. The fibers from these cortical neurons travel in the centrum semiovale lateral to the leg fibers and medial to the facial fibers. They go through the posterior limb of the internal capsule, anterior to the leg fibers and posterior to the face fibers, and continue caudally through the midbrain, pons, and upper medulla. At the lower medulla, these fibers cross to the contralateral side and then continue caudally in the lateral aspect of the spinal cord until they reach the motor neuron in the cervical spinal cord enlargement. The cortical neurons control voluntary movements. Brainstem and cerebellar neurons that influence arm movements are located in the red nucleus (midbrain) and in the pontine, medullary reticular formation, and cerebellar cortex.

Figure 202.1.— Schematic representation of the motor systems of the face, arms, and legs, and central and peripheral nervous systems structures involved in limb movements. The colored rectangles indicate the location of weakness produced by damage to the different components of the somatic motor system. U: upper motor neurons; V: ventricles; T: thalamus; UQ: upper quadrant; FN: facial nerve; LQ: lower quadrant; L: lower motor neurons; BP: brachial plexus; LSP: lumbosacral plexus.